enerated　is　indestructible；　and　the　indestructible　ungenearted；　then　each　of　them　is　coincident　with　'eternal'；　anything　ungenerated　is　eternal　and　anything　indestructible　is　eternal。　This　is　clear　too　from　the　definition　of　the　terms；　Whatever　is　destructible　must　be　generated；　for　it　is　either　ungenerated；　or　generated；　but；　if　ungenerated；　it　is　by　hypothesis　indestructible。　Whatever；　further；　is　generated　must　be　destructible。　For　it　is　either　destructible　or　indestructible；　but；　if　indestructible；　it　is　by　hypothesis　ungenerated。　　　If；　however；　'indestructible'　and　'ungenerated'　are　not　coincident；　there　is　no　necessity　that　either　the　ungenerated　or　the　indestructible　should　be　eternal。　But　they　must　be　coincident；　for　the　following　reasons。　The　terms　'generated'　and　'destructible'　are　coincident；　this　is　obvious　from　our　former　remarks；　since　between　what　always　is　and　what　always　is　not　there　is　an　intermediate　which　is　neither；　and　that　intermediate　is　the　generated　and　destructible。　For　whatever　is　either　of　these　is　capable　both　of　being　and　of　not　being　for　a　definite　time：　in　either　case；　I　mean；　there　is　a　certain　period　of　time　during　which　the　thing　is　and　another　during　which　it　is　not。　Anything　therefore　which　is　generated　or　destructible　must　be　intermediate。　Now　let　A　be　that　which　always　is　and　B　that　which　always　is　not；　C　the　generated；　and　D　the　destructible。　Then　C　must　be　intermediate　between　A　and　B。　For　in　their　case　there　is　no　time　in　the　direction　of　either　limit；　in　which　either　A　is　not　or　B　is。　But　for　the　generated　there　must　be　such　a　time　either　actually　or　potentially；　though　not　for　A　and　B　in　either　way。　C　then　will　be；　and　also　not　be；　for　a　limited　length　of　time；　and　this　is　true　also　of　D；　the　destructible。　Therefore　each　is　both　generated　and　destructible。　Therefore　'generated'　and　'destructible'　are　coincident。　Now　let　E　stand　for　the　ungenerated；　F　for　the　generated；　G　for　the　indestructible；　and　H　for　the　destructible。　As　for　F　and　H；　it　has　been　shown　that　they　are　coincident。　But　when　terms　stand　to　one　another　as　these　do；　F　and　H　coincident；　E　and　F　never　predicated　of　the　same　thing　but　one　or　other　of　everything；　and　G　and　H　likewise；　then　E　and　G　must　needs　be　coincident。　For　suppose　that　E　is　not　coincident　with　G；　then　F　will　be；　since　either　E　or　F　is　predictable　of　everything。　But　of　that　of　which　F　is　predicated　H　will　be　predicable　also。　H　will　then　be　coincident　with　G；　but　this　we　saw　to　be　impossible。　And　the　same　argument　shows　that　G　is　coincident　with　E。　　　Now　the　relation　of　the　ungenerated　（E）　to　the　generated　（F）　is　the　same　as　that　of　the　indestructible　（G）　to　the　destructible　（H）。　To　say　then　that　there　is　no　reason　why　anything　should　not　be　generated　and　yet　indestructible　or　ungenerated　and　yet　destroyed；　to　imagine　that　in　the　one　case　generation　and　in　the　other　case　destruction　occurs　once　for　all；　is　to　destroy　part　of　the　data。　For　（1）　everything　is　capable　of　acting　or　being　acted　upon；　of　being　or　not　being；　either　for　an　infinite；　or　for　a　definitely　limited　space　of　time；　and　the　infinite　time　is　only　a　possible　alternative　because　it　is　after　a　fashion　defined；　as　a　length　of　time　which　cannot　be　exceeded。　But　infinity　in　one　direction　is　neither　infinite　or　finite。　（2）　Further；　why；　after　always　existing；　was　the　thing　destroyed；　why；　after　an　infinity　of　not　being；　was　it　generated；　at　one　moment　rather　than　another？　If　every　moment　is　alike　and　the　moments　are　infinite　in　number；　it　is　clear　that　a　generated　or　destructible　thing　existed　for　an　infinite　time。　It　has　therefore　for　an　infinite　time　the　capacity　of　not　being　（since　the　capacity　of　being　and　the　capacity　of　not　being　will　be　present　together）；　if　destructible；　in　the　time　before　destruction；　if　generated；　in　the　time　after　generation。　If　then　we　assume　the　two　capacities　to　be　actualized；　opposites　will　be　present　together。　（3）　Further；　this　second　capacity　will　be　present　like　the　first　at　every　moment；　so　that　the　thing　will　have　for　an　infinite　time　the　capacity　both　of　being　and　of　not　being；　but　this　has　been　shown　to　be　impossible。　（4）　Again；　if　the　capacity　is　present　prior　to　the　activity；　it　will　be　present　for　all　time；　even　while　the　thing　was　as　yet　ungenerated　and　non…existent；　throughout　the　infinite　time　in　which　it　was　capable　of　being　generated。　At　that　time；　then；　when　it　was　not；　at　that　same　time　it　had　the　capacity　of　being；　both　of　being　then　and　of　being　thereafter；　and　therefore　for　an　infinity　of　time。　　　It　is　clear　also　on　other　grounds　that　it　is　impossible　that　the　destructible　should　not　at　some　time　be　destroyed。　For　otherwise　it　will　always　be　at　once　destructible　and　in　actuality　indestructible；　so　that　it　will　be　at　the　same　time　capable　of　always　existing　and　of　not　always　existing。　Thus　the　destructible　is　at　some　time　actually　destroyed。　The　generable；　similarly；　has　been　generated；　for　it　is　capable　of　having　been　generated　and　thus　also　of　not　always　existing。　　　We　may　also　see　in　the　following　way　how　impossible　it　is　either　for　a　thing　which　is　generated　to　be　thenceforward　indestructible；　or　for　a　thing　which　is　ungenerated　and　has　always　hitherto　existed　to　be　destroyed。　Nothing　that　is　by　chance　can　be　indestructible　or　ungenerated；　since　the　products　of　chance　and　fortune　are　opposed　to　what　is；　or　comes　to　be；　always　or　usually；　while　anything　which　exists　for　a　time　infinite　either　absolutely　or　in　one　direction；　is　in　existence　either　always　or　usually。　That　which　is　by　chance；　then；　is　by　nature　such　as　to　exist　at　one　time　and　not　at　another。　But　in　things　of　that　character　the　contradictory　states　proceed　from　one　and　the　same　capacity；　the　matter　of　the　thing　being　the　cause　equally　of　its　existence　and　of　its　non…existence。　Hence　contradictories　would　be　present　together　in　actuality。　　　Further；　it　cannot　truly　be　said　of　a　thing　now　that　it　exists　last　year；　nor　could　it　be　said　last　year　that　it　exists　now。　It　is　therefore　impossible　for　what　once　did　not　exist　later　to　be　eternal。　For　in　its　later　state　it　will　possess　the　capacity　of　not　existing；　only　not　of　not　existing　at　a　time　when　it　exists…since　then　it　exists　in　actuality…but　of　not　existing　last　year　or　in　the　past。　Now　suppose　it　to　be　in　actuality　what　it　is　capable　of　being。　It　will　then　be　true　to　say　now　that　it　does　not　exist　last　year。　But　this　is　impossible。　No　capacity　relates　to　being　in　the　past；　but　always　to　being　in　the　present　or　future。　It　is　the　same　with　the　notion　of　an　eternity　of　existence　followed　later　by　non…existence。　In　the　later　state　the　capacity　will　be　present　for　that　which　is　not　there　in　actuality。　Actualize；　then；　the　capacity。　It　will　be　true　to　say　now　that　this　exists　last　year　or　in　the　past　generally。　　　Considerations　also　not　general　like　these　but　proper　to　the　subject　show　it　to　be　impossible　that　what　was　formerly　eternal　should　later　be　destroyed　or　that　what　formerly　was　not　should　later　be　eternal。　Whatever　is　destructible　or　generated　is　always　alterable。　Now　alteration　is　due　to　contraries；　and　the　things　which　compose　the　natural　body　are　the　very　same　that　destroy　it。
  Book　II　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　1
  THAT　the　heaven　as　a　whole　neither　came　into　being　nor　admits　of　destruction；　as　some　assert；　but　is　one　and　eternal；　with　no　end　or　beginning　of　its　total　duration；　containing　and　embracing　in　itself　the　infinity　of　time；　we　may　convince　ourselves　not　only　by　the　arguments　already　set　forth　but　also　by　a　consideration　of　the　views　of　those　who　differ　from　us　in　providing　for　its　generation。　If　our　view　is　a　possible　one；　and　the　manner　of　generation　which　they　assert　is　impossible；　this　fact　will　have　great　weight　in　convincing　us　of　the　immortality　and　eternity　of　the　world。　Hence　it　is　well　to　persuade　oneself　of　the　truth　of　the　ancient　and　truly　traditional　theories；　that　there　is　some　immortal　and　divine　thing　which　possesses　movement；　but　movement　such　as　has　no　limit　and　is　rather　itself　the　limit　of　all　other　movement。　A　limit　is　a　thing　which　contains；　and　this　motion；　being　perfect；　contains　those　imperfect　motions　which　have　a　limit　and　a　goal；　having　itself　no　beginning　or　end；　but　unceasing　through　the　infinity　of　time；　and　of　other　movements；　to　some　the　cause　of　their　beginning；　to　others　offering　the　goal。　The　ancients　gave　to　the　Gods　the　heaven　or　upper　place；　as　being　alone　immortal；　and　our　present　argument　testifies　that　it　is　indestructible　and　ungenerated。　Further；　it　is　unaffected　by　any　mortal　discomfort；　and；　in　addition；　effortless；　for　it　needs　no　constraining　necessity　to　keep　it　to　its　path；　and　prevent　it　from　moving　with　some　other　movement　more　natural　to　itself。　Such　a　constrained　movement　would　necessarily　involve　effort　the　more　so；　the　more　eternal　it　were…and　would　be　inconsistent　with　perfection。　Hence　we　must　not　believe　the　old　tale　which　says　that　the　world　needs　some　Atlas　to　keep　it　safe…a　tale　composed；　it　would　seem；　by　men　who；　like　later　thinkers；　conceived　of　all　the　upper　bodies　as　earthy　and　endowed　with　weight；　and　therefore　supported　it　in　their　fabulous　way　upon　animate　necessity。　We　must　no　more　believe　that　than　follow　Empedocl